Mixture is better: enhanced electrochemical performance of phenyl selenosulfide in rechargeable lithium batteries

Article abstract of DOI:10.1039/c8cc04076a

Phenyl selenosulfide (PhS-SePh) is synthesized by an exchange reaction between phenyl disulfide and diselenide. PhS-SePh possesses average values of lattice parameters, bond character, and the lowest unoccupied molecular orbital as expected, but displays a higher discharge voltage plateau and much better cycling stability than the two precursors in rechargeable lithium batteries.

Full text of DOI:10.1039/c8cc04076a

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DOI: 10.1039/C8CC04076A
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Mixture is Better: Enhanced Electrochemical Performance of
Phenyl Selenosulfide in Rechargeable Lithium Batteries
Wei Guo,a Amruth Bhargav,b Joseph D. Ackerson,c Yi Cui,d,e Ying Ma,c* and Yongzhu Fua*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
ring at room temperature. The sulfur-sulfur (S-S) bonds break to
form lithium polysulfide Li2Sx (x = 2-8) when electrochemically
reduced in lithium batteries.2 However, sulfur and its reduced
compounds are not conductive, therefore a significant amount
of carbon in the electrode is needed. Recently, selenium as an
electrode material in lithium and sodium batteries has attracted
a lot of attention because of its higher electrical conductivity
(10-5 S cm-1) and mass density (4.82 g cm-3).7-15 But elemental
selenium has a low theoretical specific capacity of 675 mAh g-1
due to its large molecular weight. Inorganic S-Se mixture
compounds such as SxSey with various atomic ratios have shown
interesting properties and performance in rechargeable lithium
and sodium batteries,7,16-21 for example their specific capacities
are higher than that of selenium and their conductivity is better
than that of sulfur. SxSey undergoes an intertwined reduction
process forming discharge products of Li2S and Li2Se, resulting
in a complicated conversion reaction. In addition, SxSey has a
disordered structure and the bond breaking and formation in
SxSey are random,22 which add complexity in understanding the
S-Se bonds in batteries. Recently, organopolysulfides have been
studied as cathode materials in lithium batteries and shown
unique properties such as ease of synthesis, structural
tunability, and precise active sites.23-25 They were also used as
additives in electrolyte or solid-electrolyte interphase.26,27
These merits make them useful for studying S-S, and potential
Se-Se and S-Se bonds in lithium and other battery systems.
Herein, we select phenyl selenosulfide (PhS-SePh), an organic
compound containing a single S-Se bond to understand its
property and electrochemical behavior in rechargeable lithium
batteries. Phenyl disulfide (PhS-SPh) and diselenide (PhSe-SePh)
were selected as precursors and control compounds. They could
undergo an exchange reaction to form PhS-SePh, as shown in
Figure 1a.23 PhS-SePh can also be synthesized by chloramination
reactions with mixtures of thiols and selenols, which involve
multiple steps of synthesis and purification.28 Our calculation
shows the energy change for the exchange reaction is slightly
negative, which is consistent with data in literature,22 indicating
a spontaneous reaction. The theoretical specific capacity of PhS-
SePh falls in between those of PhS-SPh and PhSe-SePh when 2e-
transfer is considered for all three compounds. The reaction is
straightforward, where equimolar PhS-SPh and PhSe-SePh were
Phenyl selenosulfide (PhS-SePh) is synthesized by an exchange
reaction between phenyl disulfide and diselenide. PhS-SePh
possesses average values of lattice parameter, bond character, and
the lowest unoccupied molecular orbital as expected, but displays
higher discharge voltage plateau and much better cycling stability
than the two precursors in rechargeable lithium batteries.
Electrical energy storage has become unprecedentedly
important for advanced portable electronics and electric
vehicles. Lithium-ion (Li-ion) batteries have the highest specific
energy and energy density among rechargeable battery
systems, depending on the ion intercalation mechanism which
has the advantages of high reversibility and high voltage in
transition metal oxides such as LiCoO2.1 Driven by the demand
of high energy batteries, alternative cathode materials such as
sulfur and oxygen with high specific capacities have been
actively pursued in recent years.2-5 The ion storage mechanism
relies on the conversion reactions, e.g., sulfur to lithium sulfide,
which typically involve frequent bond breaking and formation
resulting in huge volume increases, dramatic structural
transitions, and phase changes.6 These complicated conversion
processes result in the instability of cycling performance,
therefore rendering them unsuitable for practical applications.
To utilize the high capacities of these materials, our
understanding in chemical bonds and their behavior in lithium
batteries needs to be advanced. Therefore, new approaches
could be developed to tackle the intrinsic issues preventing
their widespread application.
Elemental sulfur has a theoretical specific capacity of 1675
mAh g-1 and forms a crystal structure composed of octa-atom
a.College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou 450001, China. E-mail address: yfu@zzu.edu.cn (Y. Fu)
b.Materials Science and Engineering, University of Texas at Austin, TX 78712,
United States
c. Materials Science and Engineering, University of Wisconsin-Eau Claire, WI 54702,
United States. E-mail address: yingma@uwec.edu (Y. Ma)
d.Department of Mechanical Engineering, Indiana University-Purdue University
Indianapolis, Indianapolis, IN 46202, United States
e. School of Mechanical Engineering, Purdue University, West Lafayette, IN 46907,
United States
† Electronic Supplementary Information (ESI) available: Materials and methods,
calculation, and additional electrochemical data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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mixed in dimethoxyethane and stirred for 30 min. PhS-SePh was min are seen. The peak at 14.6 min is attributed VtioewPAhrtiScl-eSOenPlihne.
formed once the solvent was removed. The scanning
The corresponding mass spectrum is shDoOwIn: 10in.10F3i9g/uCr8eC1Cd04. 0T7h6eA
parent ion peak at m/z of 266.0 corresponds to the molar mass
of PhS-SePh. Other daughter fragmentation peaks
corresponding to that of PhS-Se· (phenyl sulfoselenide radical,
m/z = 186.0), PhSe· (phenyl selenide radical, m/z = 157.0), ·S-Se·
(sulfoselenide radical, m/z = 109.0) and PhS· (phenyl sulfide
radical, m/z = 77.1) are also evident. The peaks at 14.1 and 15.1
min can be assigned to PhS-SPh and PhSe-SePh, respectively,
based on the m/z ratios in Figure S6. The presence of two
precursors is believed to be due to the heating injection process
which causes disproportionation of PhS-SePh into PhS-SPh and
PhSe-SePh. We also performed nuclear magnetic resonance
(NMR). Figure S7 shows the 13C-NMR spectra of these
compounds, which also supports the success of making PhS-
SePh via this route.
a
b
S
Se
S
R.T.
S
Se
Se
+
-1
-1
-1
202.1 mAh g
245.5 mAh g
171.7 mAh g
PhS-SPh
PhSe-SePh
PhS-SePh
c
d
(011)
266.0
PhS-SPh
PhSe-SePh
PhS-SePh
186.0
157.0
109.0
77.1
To understand the S-Se bond nature in PhS-SePh in catholyte
solution, we calculate the lowest unoccupied molecular orbital
(LUMO), the highest occupied molecular orbital (HOMO), and
bond energies using an implicit solvation model. Figure 2a
shows the LUMO/HOMO energy levels of the three compounds.
The LUMO are in the order of PhSe-SePh (-0.99 eV) < PhS-SePh
(-0.86 eV) < PhS-SPh (-0.69 eV). For molecules with similar
structures, LUMO levels correlate with the reduction potential
in batteries, and a lower LUMO energy corresponds to a higher
reduction potential.31 It is thus expected that the discharge
voltage plateau of these compounds would follow the reverse
order. Figure 2b shows the bond energies of the three
compounds. The S-S bond has a higher bond energy than that
of the Se-Se bond because of the higher electronegativity and
smaller ionic radius of sulfur compared to selenium. The S-Se
bond energy stays in the middle of the S-S and Se-Se bond
energies as expected. The energy changes of lithiation reactions
of PhS-SPh, PhSe-SePh, and PhS-SePh are shown in Table S2.
To evaluate the electrochemical behavior of these
compounds, 0.5 M catholytes were prepared and tested in
bulky paper current collector which has been successfully used
in our previous studies.31 Figure 3a shows the cyclic
voltammogram (CV) of three cells with PhS-SPh, PhSe-SePh, and
PhS-SePh catholytes. All of them show single cathodic and
anodic peaks. PhSe-SePh shows fast kinetics (i.e., small peak
separation) compared to PhS-SPh and PhS-SePh due to the low
Se-Se bond energy. The onset cathodic potentials are in the
order of PhS-SPh < PhSe-SePh < PhS-SePh and the onset anodic
potentials are in the order of PhSe-SePh = PhS-SePh < PhS-SPh.
Figure 3b shows the voltage profiles of these cells, which exhibit
single discharge and charge voltage plateaus. The discharge
voltage plateaus are in the order of PhS-SPh (2.1 V) < PhSe-SePh
(2.2 V) < PhS-SePh (2.22 V). It is interesting that the
50
100
150
200
250
300
11.0
11.5
12.0 21
22
23
24
2 ()
2 (
m/z
Figure 1 (a) Scheme of reaction of phenyl disulfide (PhS-SPh) and phenyl diselenide
(PhSe-SePh) to form phenyl selenosulfide (PhS-SePh) along with their specific
capacities. (b) View of the unit cell of PhS-SePh along c axis. (c) X-ray diffraction
patterns of three compounds. (d) Mass spectrum of the synthesized PhS-SePh.
electron microscopy (SEM) image and elemental mappings are
shown in Figure S1, which shows that the sulfur and selenium
atomic ratios are almost the same in the prepared sample.
Based on our calculation, the crystal structure of PhS-SePh is the
same as those of PhS-SPh and PhSe-SePh except lattice
parameters are the average values. For example, the calculated
c value is 23.125 Å, whereas the c values of PhS-SPh and PhSe-
SePh are 22.924 Å and 23.450 Å,29 respectively, as shown in
Table S1. The view along the a axis of the crystal structure of
PhS-SePh is shown in Figure 1b. Sulfur and selenium moieties
form one-dimensional lines which are separated by phenyl
rings. S-Se moieties are close in pair, but far from others in the
structure.
X-ray diffraction (XRD) was used to confirm the crystal
structure of the synthesized PhS-SePh. The full XRD patterns are
shown in Figure S2, which resemble the calculated ones in
Figure S3. Figure 1c shows XRD patterns in the 2θ ranges of 11-
12° and 21-24° where distinct separations are observed. It can
be seen that PhS-SPh and PhSe-SePh show a single peak of (011)
plane at 11.5° and 11.3°, respectively, PhS-SePh also shows a
single peak of (011) plane at 11.4°. In addition, all compounds
show a set of four peaks of (020), (021), (006), and (022) planes
in the 21-24° region. All the peaks of PhS-SePh fall in between
those of PhS-SPh and PhSe-SePh. The XRD results indicate the
synthesized PhS-SePh compound is quite pure since no other
impurity or overlapped peaks are observed. In addition, the
crystal structure of PhS-SePh is the same as those of PhS-SPh
and PhSe-SePh, which confirms the accuracy of the predicted
crystal structure shown in Figure 1b. This proposed crystal
structure is further validated through Fourier transform
infrared (FTIR) spectroscopy shown in Figure S4. When looking
at the 440-485 cm-1 region, the out of plane phenyl ring twist
occurring at 456 cm-1 for PhS-SePh is in between that of PhS-
SPh (463 cm-1) and PhSe-SePh (455 cm-1). Furthermore, the
transition of symmetric S-S stretch from 472.6 cm-1 in PhS-SPh
to the asymmetric stretch of the S-Se bond in PhS-SePh leading
to a doublet at 471.6 cm-1 and 477 cm-1 can be observed.30
To further confirm the success of forming PhS-SePh via this
synthesis route, gas chromatography-mass spectrometry (GC-
MS) were performed. Figure S5 presents the GC spectrum of
PhS-SePh. Three peaks at retention times of 14.1, 14.6, and 15.1
a
b
2
0
2.2
2.0
1.8
1.6
-0.69
-0.86
LUMO
HOMO
-0.99
2.01
-2
-4
1.85
-6
1.70
-7.53
-7.64
-7.77
-8
PhSe-SePh PhS-SePh
PhS-SPh
Se-Se
S-Se
S-S
-10
Figure 2 (a) Lowest unoccupied molecular orbital (LUMO) and highest occupied
molecular orbital (HOMO) energy levels of PhS-SPh, PhSe-SePh, and PhS-SePh. (b)
Sulfur-sulfur (S-S), selenium-selenium (Se-Se), and sulfur-selenium (S-Se) bond
energies in these compounds.
2 | J. Name., 2012, 00, 1-3
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a
0.50
b
3.0
2.5
2.0
1.5
View Article Online
DOI: 10.1039/C8CC04076A
PhS-SPh
PhSe-SePh
PhS-SePh
a
b
0.25
0.00
PhS-SPh
PhSe-SePh
PhS-SePh
-0.25
-0.50
PhS-SPh
PhSe-SePh
2.0
2.5
3.0
0
50
100
150
200
Potential (V vs. Li/Li+)
Specific capacity (mAh g-1
)
c
d 600
Rct
c
d
RΩ
200
150
100
50
200
150
100
50
500
400
300
200
100
0
100
100
0.5 M PhS-SPh
0.5 M PhSe-SePh
0.5 M PhS-SePh
1.0 M PhS-SPh
1.0 M PhSe-SePh
1.0 M PhS-SePh
CPEdl
80
60
40
20
0
80
60
40
20
0
PhS-SPh
PhSe-SePh
PhS-SePh
PhS-SePh
0
100
200
300
400
500
600
Z' (ohm)
0
0
Figure 4 (a) SEM images of (a) PhS-SPh, (b) PhSe-SePh, and (c) PhS-SePh electrodes
after discharge, the scale bar is 1 µm. (d) Nyquist plots of these cells, the PhS-SPh
plot fits to the equivalent circuit model shown in the figure.
0
50
100
Cycle number
150
200
0
50
100
Cycle number
150
200
Figure 3 (a) Cyclic voltammogram, (b) voltage profiles, and (c) cycling performance
of 0.5 M PhS-SPh, PhSe-SePh, and PhS-SePh catholytes, the cells were cycled at
C/5. (d) Cycling performance of 1.0 M catholytes of these compounds at C/5.
stability than the 0.5 M PhSe-SePh catholyte but high capacity
fade persists. In contrast, PhS-SePh shows the highest capacity
and best cycling stability over 200 cycles. Selected voltage
profiles are presented in Figure S10. In addition, the Coulombic
efficiencies of the two cells with PhS-SePh catholytes are
>99.5%, representing high reversibility. The battery
performance results prove the mixture PhS-SePh is the best
cathode material among three compounds in rechargeable
lithium batteries. Additionally, the density of PhS-SePh is 1.597
g cm-3, which is between that of PhS-SPh (1.353 g cm-3) and
PhSe-SePh (1.84 g cm-3). Based on the performance shown
above, the specific energy is estimated to be 337 Wh kg-1 and
the energy density is 538 Wh L-1.
Figures 4 a-c show the SEM images of the three electrodes
after discharge. PhS-SPh and PhSe-SePh electrodes look similar,
in which agglomeration is seen and some carbon fibers are bare.
In contrast, the PhS-SePh electrode is quite porous, in which all
carbon fibers are coated with the discharged products. The
observed morphology of the discharged electrodes supports
the entropy effect claimed above. Due to the mixing process of
PhS-Li and PhSe-Li being kinetically slower, uniform growth is
observed. However, growth of the single solid phase of either
PhSe-Li or PhS-Li is much faster and thus results in their
agglomeration. Furthermore, this agglomeration of non-
conductive PhS-Li results in the highest voltage hysteresis. In
contrast, PhSe-Li shows much lower voltage hysteresis even
though it agglomerates, which is believed to be due to the
conductivity of Se. The mixture of PhS-Li and PhSe-Li is in good
contact with conductive carbon nanofibers, therefore the
voltage hysteresis is low. In addition to voltage hysteresis, this
phenomenon can also explain the better longevity of PhS-SePh
cells. The better adhesion of the mixed discharge products to
the carbon substrate and the need to form S-Se bonds on
charge minimizes the detachment and also dissolution of these
particles leading to reduced shuttle effect and thus prolonged
cycle life.
discharge voltage plateau of PhS-SePh is slightly higher than
that of PhSe-SePh, which is consistent with the CV result in
Figure 3a but inconsistent with the calculated LUMO levels
shown in Figure 2a. We postulate that this anomaly can be
explained through thermodynamics. It is known that the
reaction voltage can be determined from the difference in the
Gibbs free energy (ΔG = ΔH - TΔS) between the reactant and the
product. Given that the energy between S-Se bond is very close
to that of pure S-S and Se-Se bonds, it is reasonable to expect
little to no change in the enthalpy term. However, the mixing of
the discharged products, PhSe-Li and PhS-Li could result in the
relatively larger entropy term in the case of PhS-SePh which
would be absent in the case of PhSe-SePh and PhS-SPh since
only a single product, either PhSe-Li or PhS-Li, is formed. The
resultant lowering of Gibbs free energy for the reaction
products leads to higher reaction voltage observed in PhS-SePh.
The charge voltage plateaus are in the order of PhSe-SePh <
PhS-SePh < PhS-SPh. PhS-SPh shows the highest voltage
hysteresis which has been observed in prior literature,32,33 while
PhSe-SePh shows the lowest, which is related to the bond
energy resulting in different overpotential.
Figure 3c shows the cycling performance. The initial
capacities are in the order of PhSe-SePh (141 mAh g-1) < PhS-
SePh (152 mAh g-1) < PhS-SPh (185 mAh g-1), which are
consistent with their theoretical values. However, the capacities
of PhS-SPh and PhSe-SePh degrade very fast and fall below 50
mAh g-1 after 100 cycles, which is due to the solubility of PhSLi
and PhSeLi in the electrolyte that leads to the loss of active
material and consequently shorter cycle life. In contrast, PhS-
SePh shows a much better cycling performance with a capacity
retention of 52% (79 mAh g-1) over 200 cycles. The cell also
shows good rate performance (Figure S8). To exclude the effect
of catholyte concentration, 1.0 M catholytes were also
evaluated. Figure S9 shows the voltage profiles of the three
compounds, clearly PhS-SePh has the highest discharge voltage
plateau. More interestingly, PhS-SePh shows the highest initial
discharge capacity which is partially due to the incomplete
discharge of PhS-SPh because of the high ohmic overpotential.
Figure 3d shows the cycling performance. Again, the discharge
capacity of PhS-SPh degrades very fast. However, PhSe-SePh
shows better cycling
This phenomenon was further explored through
electrochemical impedance spectroscopy (EIS). Figure 4d shows
Nyquist plots of these cells after discharge. The intercepts in the
real axis in the high frequency range are the bulk resistance and
resistance of electrodes (RΩ). The bulk resistance of liquid
electrolyte can be considered to be the same in these cells.
Therefore, the intercept difference is due to the resistance of
electrodes which is in the order of PhSe-SePh (5 ohms) < PhS-
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DOI: 10.1039/C8CC04076A
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YF acknowledges the support from the Recruitment
Program of Global Youth Experts in China. JA and YM would like
to acknowledge the support from Materials Science and
Engineering at the University of Wisconsin-Eau Claire, as well as
the computational resource provided by the Blugold
Supercomputing Cluster.
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Conflicts of Interest
There are no conflicts to declare.
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Notes and references
4 | J. Name., 2012, 00, 1-3
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